This invention relates to ultrasonic welding of sheet material, and more particularly to a method and apparatus for ultrasonically welding, in overlapped relationship, a number of composite sheets.
The term "composite sheet" or "composite material" as herein used includes the so-called processed paper having a base layer of paper or paper-like scorchable material such as cellophane and an outer coated layer of a thermoplastic synthetic resin material, and a sheet or film material having a continuous phase of a thermoplastic synthetic resin material and a dispersed phase of an inorganic material.
There are several known methods and apparatus which utilize an ultrasonic vibrator or ultrasonic energy for bonding a number of sheets of a thermoplastic synthetic resin or uncoated foils of aluminum.
In the methods and apparatus for bonding or welding heretofore available, it has been a usual practice that at least one of vibrator tool or working table used therein has to be contoured to follow the configuration of the object to be bonded or welded.
This aims at maintaining the configuration of the objects to be bonded or welded in its original or predetermined form.
Such vibrator tool and working table are disclosed, for example, in U.S. Pat. Nos. 3,438,824, 3,386,870, 3,436,006, 3,671,366 and 3,440,118.
Yet, those are applicable only in bonding plastic materials.
There has been an increasing tendency in the packaging industry to utilize the processed paper or composite sheets as material for fabricating packaging containers for liquids, creams or other fluidized materials. When sealing such containers holding a liquid or fluid therein, it has been the conventional practice to employ an external heating means such as a gas burner or an electric heat irradiator. However, the employment of an external heating means is disadvantageous to a large-scaled production as it requires high production costs and complicated operational procedures which can be coped with only by high technical skills. Moreover, seals formed by an external heating means are generally insecure and less than satisfactory in appearance.
Many attempts have thus far been made to ultrasonically seal the packaging containers of the composite material of the nature as mentioned above. The prior art structure and methods have met with various difficulties since the composite sheet includes a layer of paper-like material which is thermally non-conductive and easily scorchable.
With a thermoplastic synthetic resin film dispersed with particles of an inorganic material, pin holes are formed in the film upon application of ultrasonic vibrations as the thermoplastic resin material is melted by the applied ultrasonic energy, dislodging the dispersed particles of the inorganic material. The scorches and pin holes are developed in an increased degree particularly where the sealing portion of the container has a varying thickness with a greater number of plies of the composite sheet in certain localities than in other localities. This is because the applied static pressure is unevenly distributed to the thicker and thinner portions of the overlapped sheets and the applied ultrasonic energy propagates non-uniformly into the overlapped sheets in the welding portion of the container, generating high scorching heat in particular localities of the welding portion by localized concentrations of the ultrasonic energy. For these reasons, it has been considered to seal the containers of the composite paper sheets by the use of the existing ultrasonic welding techniques.